This is a bit of a hard one to answer because people might have their own answer to it, but for me the answer has to be The Maze of Lost Souls, that hides the way to Demona. With its multiple levels and one way drops, moving around in there can be hazardous and confusing. Remembering this was implemented way back in June 10 2001, when the current mapping abilities that we have now did not exist it could be deadly. Ive spoken to 6-7 people about times back in the early days and they would be lost in there for 2 or 3 days and their party would just end up committing suicide to escape. The one time I went in I spent a day in there and just luckily managed to stumble over the exit.
Right now I have been mapping the MOLS after returning to the game, using all the mapping features that we have today and while it is still much easier, I still have been struggling to finish the map. One particular area is left to complete, but even using the current mapping features, and having been to this area at least once before, I can not find my way back to it to finish it. I shall persevere.
It also feels that MOLS was something of a place holder that was never used as there are a couple of place that you cant pass so theres still a couple of mysteries down there.